This invention relates to fuel cell systems for powering electric vehicles that are used over a wide range of ambient and operating conditions, and more particularly to optimizing the performance of such systems under such conditions by actively varying the system""s backpressure.
Fuel cells in general, and PEM fuel cells in particular, have been proposed for use as electrical power plants to replace internal combustion engines. PEM fuel cells are well known in the art, and include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode catalyst layer on one of its faces and a cathode catalyst layer on the opposite face. The MEA is sandwiched between a pair of electrically conductive current collectors which also serve to distribute hydrogen to the anode and oxygen (i.e. from compressed air) to the cathode. The H2 and O2 react to form water that exits the fuel cell primarily as part of the cathode exhaust (a.k.a. cathode tailgas). The cathode/air feedstream (and sometimes the anode/H2 stream) is typically humidified to keep the ion-exchange membrane from drying out.
Some fuel cell systems use pressurized, or liquid, hydrogen to fuel the fuel cell. Others store the hydrogen chemically as a thermally dissociable hydride, or physiochemically by heat-releasable adsorption on a suitable adsorbent (e.g. carbon nanofibers). Still others dissociate hydrogenous liquids such as gasoline, methanol, or the like to provide the hydrogen used by the fuel cell. To release their hydrogen, hydrogenous liquids are dissociated in a so-called xe2x80x9cfuel processorxe2x80x9d. One known fuel processor for dissociating gasoline, for example, is a two stage primary reactor known as an xe2x80x9cautothermal reformerxe2x80x9d. In an autothermal reformer, gasoline and water vapor (i.e. steam) are mixed with air and pass sequentially through two reaction sections, i.e. a first xe2x80x9cpartial oxidationxe2x80x9d (POX) section, and a second xe2x80x9csteam reformingxe2x80x9d (SR) section. In the POX section, the gasoline reacts exothermically with a substoichiometric amount of air to produce carbon monoxide, hydrogen and lower hydrocarbons (e.g. methane). The hot POX reaction products pass into the SR section where the lower hydrocarbons react with the steam to produce a reformate gas comprising principally hydrogen, carbon dioxide, carbon monoxide, water methane and nitrogen. One such autothermal reformer is described in International Patent Publication Number WO 98/08771 published Mar. 5, 1998. The process of producing hydrogen from methanol is similar to that used for gasoline wherein the primary reactor can either be (1) POX only, (2) POX+SR, or (3) SR only. One known fuel processor for dissociating methanol is a steam reformer such as described in U.S. Pat. No. 4,650,727 to Vanderborgh. In both cases, the steam reformers require water as one of the reactants.
The carbon monoxide concentration in the reformate exiting a primary reactor is too high for the reformate to be used in a fuel cell without poisoning it. Accordingly, most fuel processors include a downstream section for cleansing the reformate of CO by subjecting it to CO separation membranes, to CO-adsorption media, or to a so-called xe2x80x9cwater-gas-shiftxe2x80x9d (WGS) reaction wherein water (i.e. steam) reacts exothermically with the carbon monoxide to produce CO2+H2. The WGS reaction also requires water as a reactant. A so-called PrOx (i.e. preferential oxidation) reactor may also be used downstream of the water-gas-shift reactor to remove any residual CO exiting the WGS reactor.
It is known to burn the cathode and anode tailgases exiting a fuel cell in a downstream combuster to form water, and to provide heat for use elsewhere in the system, e.g. to heat the fuel processor. Moreover, it is known that water management of fuel cell systems that are to be used for vehicular applications (i.e. cars, trucks, buses etc.) is an important consideration. In this regard, it is desirable to collect the water generated by the fuel cell system (e.g. from the combustor exhaust and/or the fuel cell tailgases) and reuse it elsewhere in the system (e.g. in the fuel processor, the water-gas-shift reactor or a humidifier) where it is needed rather than storing an extra supply of water on-board for such system needs. Optimally, the system will operate under a condition known as xe2x80x9cwater neutralityxe2x80x9dxe2x80x94that is, that the system will produce all the water that the system requires. Accordingly, it is known to provide one or more condensers at various locations within the system to condense water from the various gas streams and direct it to a water collection tank from whence it is distributed to where it is needed. The ability to effectively condense water generated by the system varies with the ambient conditions surrounding the system. Hence for example, it is more difficult to condense water at higher altitudes (i.e. lower pressure), and at higher temperatures than it is to condense water at low temperatures and high pressures. Also,the efficiency of the system, as well as that of the fuel cell itself, are affected by ambient temperature and pressure. Hence, for example the fuel cell is more efficient and can produce more power when it operates at higher pressures. Moreover, the compressor that provides compressed air to the fuel cell stack can only operate effectively within a defined range of operating parameters. In this regard, the performance of each compressor, whether it be a centrifugal or positive-displacement type compressor, is defined by a compressor performance map which (1) is a plot of the compressor pressure ratio (i.e. compressor output pressure/compressor inlet pressure) on the vertical axis versus the mass flow rate of air on the horizontal axis, and (2) shows the operating envelope where acceptable performance is possible for that particular compressor. The compressor inlet pressure is equal to the ambient pressure minus any inlet losses. That operating envelope (hereafter xe2x80x9cnormal operating envelopexe2x80x9d) is bounded by two extremes beyond which the compressor will not work effectively e.g.,. because of surge, overheating, choked flow, or some other condition that is deleterious to the compressor or its performance.
The present invention dynamically controls the system""s backpressure to optimize water recovery, system efficiency, cell performance and compressor performance under varying conditions of ambient temperatures and pressures.
The present invention comprehends a method and apparatus for optimizing the performance (e.g. electrical output, compressor efficiency, water neutrality, system efficiency, etc.) of a fuel cell system under changing ambient conditions (i.e. temperature and pressure). The invention involves a fuel cell system of the type that comprises (1) a fuel cell having an anode outlet that discharges a H2-containing anode tailgas and a cathode outlet that discharges an O2-containing cathode tailgas, (2) a hydrogen source for providing hydrogen to an anode of said fuel cell, and (3) an air compressor for providing oxygen to a cathode of said fuel cell. The system may also include one or more, condenser(s) for condensing water out of one or more of the system""s reactant streams, and a combuster for combusting electrode tailgas(es).
In accordance with one aspect of the invention, there is a method that comprises: (a) providing a modulateable pressure regulator downstream of the cathode outlet for varying the backpressure of the cathode tailgas; (b) sensing the ambient (e.g. temperature, pressure, humidity etc.) surrounding the system, and sending a signal(s) indicative thereof to a controller; (c) sensing at least one operating condition of the system (e.g. water collected, compressor inlet/outlet pressure, system backpressure, etc.) and sending a signal(s) indicative thereof to a controller; and (d) modulating the regulator, via the controller, in response to the signals to optimize the performance of the system under those ambient/operating conditions. The regulator may be positioned solely in the cathode tailgas stream, or downstream of a combuster that burns the cathode and anode tailgases before emitting them to the atmosphere. According to one embodiment of the invention, the system backpressure is increased as the ambient temperature increases to make it easier to condense water at elevated temperatures. In another embodiment of the invention, the system backpressure is increased as the ambient pressure decreases (e.g. at high elevations) to help maintain a substantially constant compressor output pressure, and hence a substantially constant operating pressure in the fuel cell. In still another embodiment of the invention, the backpressure is reduced when the compressor ratio (i.e. pressure compressor output pressure/compressor inlet pressure) is such as to cause the compressor to operate outside its acceptable operating envelope (e.g. in the surge region of a centrifugal compressor).
Another aspect of the invention involves apparatus for carrying out the aforesaid method. In this regard, there is provided a fuel cell system that includes: (1) a fuel cell having an anode outlet that discharges a H2-containing anode tailgas and a cathode outlet that discharges an O2-containing cathode tailgas; (2) a hydrogen source for providing hydrogen to an anode of the fuel cell; and (3) an air compressor for providing oxygen to a cathode of the fuel cell. In accordance with this apparatus aspect of the invention, the system further includes: (a) a modulateable pressure regulator associated with the cathode outlet for varying the backpressure of the cathode tailgas; (b) pressure sensor for sensing the compressor output pressure and sending signals indicative thereof to a controller; and (c) a controller associated with the pressure regulator and the sensor for modulating the regulator so as to vary the backpressure in response to the output pressure of the compressor such as to maintain the output pressure substantially constant (i.e. near a predetermined set point), regardless of ambient pressure. In another embodiment of the apparatus, there is further provided: (i) a combuster for burning the anode and cathode tailgases to produce a water-containing exhaust gas; (ii) an exhaust outlet from the combuster for discharging the exhaust gas from the combuster; (iii) a modulateable pressure regulator associated with the exhaust outlet for varying the backpressure of the exhaust gas; (iv) a first pressure sensor for sensing the ambient pressure and sending a signal indicative thereof to a controller; (v) a first temperature sensor for sensing the ambient temperature and sending a signal indicative thereof to a controller; (vi) a second pressure sensor for sensing the backpressure of the exhaust gas and sending a signal indicative thereof to a controller; and (vii) a controller associated with the pressure regulator for modulating the regulator so as to vary the backpressure in response to the ambient temperature/pressure and the backpressure . Another embodiment includes a sensor for sensing the compressor output pressure and the controller varies the system backpressure via the pressure regulator in response to the ambient temperature/pressure, the compressor output pressure, and the system backpressure. In still another embodiment, the system includes a condenser for condensing water out of the exhaust gas, a tank for collecting the condensed water for use within the system, a level detector associated with the tank for detecting the level of water in the tank and sending a signal indicative thereof to controller, and a controller associated with the pressure regulator for modulating the regulator so as to vary the backpressure on the system in response to the ambient temperature/pressure, the backpressure and the water level in the tank.